How to fix Warning: validateDOMNesting(…): cannot appear as a child of

Question

Most likely the component Spinner renders a <div> as the outermost node. Check the implementation of it.

You implicitly render it inside <tbody> through the lines

<tbody>
    {usersList}
</tbody>

where usersList defaults to <Spinner /> when there are no users or loading is true. This is why get the error.

A fix would be to wrap the Spinner into a td that spans the whole row:

if (users === null || loading) {
    usersList = <tr><td colSpan="4"><Spinner /></td></tr>;
} else {
    // ...
}